STEEL GRADE IS TOP OF CLASS
18 December 2002 - Sandvik Materials Technology UK

As Mark Twain might have said, the rumours of steel's death have been long exaggerated. In the 30 years since the introduction of modern stainless steel, aluminium and titanium have been in the ascendancy to an extent that many see their continued rise and the decline of steel as an engineering material as foregone conclusions.

However, Nanoflex may turn such thinking on its head. Developed by steel and tube specialist Sandvik, this ultra high strength nanotechnology alloy is said to combine the best features of steel and glass (see box insert). Until recently, such an achievement was thought impossible outside of a laboratory.

Unusually for a steel producer, Sandvik is a technology-lead company that secures its market position with a judicious balance of blue-sky research and applied development. The Nanoflex programme began with a relatively modest approach from a medical company for Sanvik to develop a new sort of suture needle for ophthalmic surgeons.

'Eyeballs are incredibly tough, but almost infinitely delicate,' says Nigel Haworth, the Sandvik man in charge of the Nanoflex project. 'Suturing them is sometimes a necessary evil, and there is a risk of causing more damage unless highly skilled surgeons use the very best needles - which have to be strong enough to penetrate the flesh yet so fine that their intrusion is barely discernable.

'We pulled together team of metallurgists, put them through a familiarisation programme with the medics, then asked them to work their magic.'

The performance brief necessitated a 10nm (nanometres) needle diameter for minimum impact on entry to the patients' flesh yet a steel strength that allows easy and reliable passage. Surprisingly the composition of the alloy is relatively simple, including: 12 per cent chromium, 9 per cent nickel, 4 per cent molybdenum and 2 per cent copper as the main components. This mix is heat treated to 450degC and thermally aged to bring out the desired qualities of strength and lightness.

'As the development of the suture needles progressed it became increasingly apparent that we had created a new material of significant importance and with potential applications in many fields,' says Nigel Haworth. 'So we created development batches of Nanoflex sheet, tube, mesh and wire and set about a massive brainstorming exercise to dream up and test as many possible applications as possible.

'It felt like we had opened Pandora's Box. Each trial application seemed to spawn yet more ideas and it became increasingly apparent how advanced our new metal will prove to be.'

In tube form alone, Nanoflex's potential for active and passive space frame applications is immense: tent poles, bike frames, pram chassis, roof trusses are just some obvious application. Shock absorbers and other active components in automotive systems could become huge markets. High pressure tubing for chemical processing, oil and gas refining, steam and gas passage are potential uses being explored by Mr Haworth.

'The medical world is very excited, so as well as watching ophthalmic surgery I have also attended cranium drilling operations and the plating together of broken bones. There are more opportunities in heart surgery and for ear nose and throat work. The police have been testing it for bullet-proof vests.'

Lightweight ice-picks are another potentially life-saving application, suggests Mr Haworth. 'If the ice breaks and you fall into the near-zero sea your life starts ticking away very fast. Climbing out onto the slippery ice is virtually impossible unless you have a couple of picks to drive into the surface!'

Haworth and his Sandvik colleagues have realised that the potential for Nanoflex is limited only by their own imaginations, so have set up a website, www.sandvik.com/nanoflex where anybody can review its properties and suggest more development paths by email.

Flexible formability

Forming a hard material is often difficult, but Sandvik Nanoflex displays excellent forming properties. In the 'as-delivered' condition (annealed or cold-worked) the material is formable. Cutting, turning, grinding and cold working - such as bending to tight radii - is said to be easy. After reaching the desired form, a simple low temperature heat treatment (precipitation hardening) gives the material its superior strength without incurring distortion.

For most high-tensile steels this is not possible, since the strength levels before and after heat treatment are relatively similar. It is the increase in tensile strength for Sandvik Nanoflex, of between 600 and 800MPa, that enables it to be formed into to complicated shapes and then to reach high tensile strength and hardness.

Ultra High-Strength

Sandvik Nanoflex combines excellent strength and hardness, yet the material is tough and malleable. Tensile strength levels between 1600 and 3000 MPa and hardness levels up to 60 HRC can be achieved; whilst maintaining a toughness of over 27J at temperatures as low as -20 degrees C.

Combined with these properties are: good corrosion and wear-resistance, high formability in the as-delivered condition, dimensional stability after heat treatment and extreme resistance to high temperatures.

Nanoflex opens up a spectrum of applications. The strength enables thin wall and thereby lightweight steel to be used in, for example, lightweight chassis and sports and outdoor equipment for extreme conditions.

http://www.smt.sandvik.com/uk

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